Brighthand reports about a new piece of software that allows to make calls using a Pocket PC and a Wi-Fi card. This is an interesting development that could turn Wi-Fi into a very disruptive technology for the mobile phone industry and may explain why companies like T-mobile are placing bets on the phenomenon, covering themselves in case other revenues (from regular mobile phones) were to evaporate. At $30/month, Wi-Fi service can currently be seen as an expensive toy for the mobile worker. But if you consider the possibility to make phone calls for the same price, the price seems low. Compared to most cellular phone programs that offer a few hundred or thousand minutes every month for roughly the same price, the idea of unmetered service could represent a huge growth opportunity for anyone offering a hotspot. However, the issue will then become one of available bandwidth. As more and more hotspots are added, the pervasiveness of Wi-Fi makes the possibility to bypass the phone network more real. The next step in that evolution would be for the Wi-Fi protocol to include some kind of mechanism to check the strength of signal. If you think of the cellular phone system,…